To enlarge capacity of an optical fiber transmission system, a digital coherent system has been in practical use. FIG. 11 is a block diagram illustrating a configuration of a general optical transmitter 800 used in a digital coherent system.
The optical transmitter 800 includes a light source 1, optical modulators 2 and 12, and a polarization multiplexer 5. The light source 1 outputs CW (continuous wave) light having a predetermined wavelength. The CW light outputted by the light source 1 is bifurcated. The bifurcated CW lights are subjected to phase modulation and intensity modulation in the optical modulators 2 and 12 by transmitted information, and become signal light. The optical modulator 2 is an optical modulator for X polarization and the optical modulator 12 is an optical modulator for Y polarization. The modulated X polarization-side signal light and the modulated Y polarization-side signal light are multiplexed in the polarization multiplexer 5 as signal light whose polarized waves are orthogonal to each other (orthogonal polarization multiplexing).
Since the configuration and operation of an optical transmitter used in a general digital coherent system are well-known, a detailed description of each unit of FIG. 11 is omitted.
FIG. 12 is a block diagram illustrating a configuration of a general optical receiver 810 used in a digital coherent system.
The optical receiver 810 includes a PBS (polarization beam splitter) 21, optical hybrid circuits 22 and 23, an AD (analog to digital) conversion unit 24, and a wavelength dispersion compensation unit 25. The optical receiver 810 further includes an adaptive equalization unit 27, a frequency offset compensation unit 28, a carrier phase estimation unit 29, and an identification determination unit 30.
Transmitted signal light is bifurcated in the PBS 21 and is mixed with local light in the optical hybrid circuits 22 and 23. The optical hybrid circuits 22 and 23 convert the signal light into analog electrical signals for output. General optical hybrid circuits 22 and 23 output analog electrical signals each having an I (in-phase) component and a Q (quadrature) component from one polarization signal. Therefore, the optical hybrid circuits 22 and 23 output four analog electrical signal in total. The analog electrical signals are converted into digital electrical signals in the AD conversion unit 24, and are outputted as a digital reception signal.
The digital reception signal is subjected to digital signal processing in the wavelength dispersion compensation unit 25, the adaptive equalization unit 27, the frequency offset compensation unit 28, the carrier phase estimation unit 29, and the identification determination unit 30, so that the transmitted signal is demodulated. The wavelength dispersion compensation unit 25 mainly compensates for static deterioration of the signal light such as wavelength dispersion of an optical transmission path. The adaptive equalization unit 27 mainly compensates for dynamic deterioration of the signal light such as polarization fluctuation. The adaptive equalization unit 27 also performs polarization division of the reception signal. The frequency offset compensation unit 28 compensates for a frequency difference between an optical carrier and local light of the optical receiver. The carrier phase estimation unit 29 compensates for a phase difference between the optical carrier and the local light. The identification determination unit 30 identifies data included in the reception signal. Since the configuration and operation of the optical receiver used for the general digital coherent system are well-known, a more detailed description of each unit of FIG. 12 is omitted.
In relation to the present invention, PTL 1 discloses a communication system including a polarization compensator in a transmitter and a receiver. PTL 2 discloses an optical communication system including a modulation system for reducing inter-polarization crosstalk. PTL 3 discloses an optical communication method for ensuring anonymity of information by polarization mode dispersion.